Most birds can fly. Most birds can walk and run and, many can swim and dive. Migrating birds fly large distances to gain access to remote habitats such as the arctic tundra and remote islands. Flight gives birds access to many food sources such as "aerial plankton", flying invertebrates and fruits of forest trees. Cliffs, islands and trees provide flying birds with nesting and roosting places where terrestrial predators cannot reach them. All birds share the same basic body plan, with minor variations.
Birds have inherited the bipedal stance of the Archosaur branch of reptiles with a number of modifications. The expanded and elongated pelvis is fused rather than articulated to the vertebrae. The bird ancestor’s bipedal stance caused the forelimb to evolve into a wing that is structurally independent of the legs. This has helped birds to evolve in such a way as to free the legs for perching, walking, running, swimming and, catching and manipulating prey. To aid in flight, birds have a different respiratory system as compared to mammals. In the absence of a diaphragm, air sacs act as a 'bellows' to ventilate the lungs. The lungs are small, with “air capillaries” through which air is drawn into a system of air sacs beyond the lungs.
Evolution of flight in birds: Evolution of the small archosaur forelimb into a gliding wing required the wing surface area to be greatly increased, while retaining sufficient strength to support the weight of the body, suspended from the shoulder joints. The arm skeleton provides the bending strength for the inner half of the wing. Beyond the hand skeleton, the keratin shafts of the primary flight feathers provide the bending strength of the hand wing. Their bases are tightly bound by connective tissue to the rudimentary hand skeleton, with no freedom of movement. The bases of the secondary flight feather shafts are bound to bumps on the back side of the ulna, with some freedom to rotate downward and inward. The surface area of the hand wing is made up of the expanded vanes of the primary feathers, while secondaries make up the area of inner part of the wing.
Smaller covert feathers smooth over the bases of flight feather, and seal the gaps between them. The free ends of each row of coverts overlap the feathers behind, in the manner of a tiled roof. All of the aerodynamic force acting on a bird’s wing is collected at the humerus, which has to support bending and twisting loads. The humerus shaft is a thin-walled, hollow cylinder, adapted to carry these loads with great efficiency. The central cavity is connected to air sac system and filled with air. Internal struts prevent buckling of the load-bearing bony wall. The primary and secondary feather shafts are hollow, and filled with a keratin to maintain the shape of the load-bearing keratin walls. The tail feathers are structurally similar to the flight feathers, with their bases attached to the rudimentary tail skeleton. They can be spread fanwise, forming an auxiliary lifting surface.
A bird’s “aerobic capacity” is determined by the capacity of its heart and lungs. Only Hummingbirds have sufficient aerobic capacity to hover continuously, although many small birds can hover anaerobically for short periods. Some large birds have insufficient aerobic capacity for sustained level flight at any speed, and are forced to to soar. While flying, birds lose heat by sending blood to thinly insulated areas of skin that are exposed to the air flow. These areas are covered when the wings are folded, thus avoiding heat loss when the bird is not flying. If the temperature is too high, the bird opens its beak and flutters the throat pouch, thereby cooling the blood by evaporating water from the upper respiratory tract. Evaporation also takes place from the lining of the air sacs, which penetrate many organs, including the interior of the pectoralis muscles.
Bird’s flight muscles work continuously, pushing the air down to produce lift that balances the weight. The power required to fly is highest when the air speed is zero (hovering), and decreases at medium and high speeds. However, additional power is required to overcome the air drag, and this increases with speed. There is a clear speed range at which the power required to fly is lower than at either slower or faster speeds. The actual available power dwindles as the size of the bird increases, thus reaching an upper limit to the mass (about 16 kg) of viable flight-capable birds. Small birds have sufficient muscle power to fly over a wide range of speeds, large birds like swans have just enough power to fly near the minimum power speed and still larger birds, such as Ostriches and Emus, are flightless..
Gulls and Ducks float on the surface of the water and use their feet in a fore-and-aft rowing motion, whereas more adapted water birds such as Loons and Grebes have the legs set far back, and swing them in a more lateral motion, using the feet as hydrofoils. Auks and Diving-petrels have wings of reduced size, forcing them to fly faster, with faster wingbeats, but they also use their wings for propulsion under water. The aquatic wing motion is quite similar to flight, but at a much reduced frequency, with the wings partly folded. Gannets, Petrels, and some Albatrosses can also swim under water in this manner to a limited extent, diving a few meters below the surface. Penguins carried this line of evolution further, with wings too small to fly, but optimized as hydrofoils. Frigate Birds do not swim or alight on the water at all, although their dispersal movements show that they spend weeks or months at a time over the open ocean, flying day and night.
Take-off: To take off, a bird needs sufficient air speed over the wings, either from forward motion, or by flapping the wings, or by a combination of both. Birds up to the size of pigeons can jump into the air from a standing start, and accelerate into flight, but larger birds have to run to get flying speed on a level surface. Swans use their large webbed feet alternately to accelerate over water, while Cormorants and Pelicans use both feet together. Large birds taking off from a tree or cliff, drop to convert height into air speed. All birds head into the wind when taking off from the ground or water.
Landing: For birds, landing into the wind is obligatory. In light winds, birds slow down when preparing to land, by increasing the frequency and amplitude of the wing beat, tilting the wings until they are beating nearly horizontally, and spreading and lowering the tail. Any residual momentum is absorbed by the legs. Glide landings are often possible in moderate wind, even for large birds. The body and wings are tilted up as the bird flares, with one final wing beat sweeping the wings forward horizontally, just before the wings are folded. Auks and Loons land on water at high speeds, lowering their bellies into the water with the feet trailing behind, whereas Ducks and Swans swing their feet forward and use them like water skis. Gannets often enter the water in a shallow dive, while Petrels and Albatrosses slow down while gliding, and drop gently onto the surface. Guillemots nest on cliff ledges and their landing technique involves diving toward the cliff at high speed, then pulling up into a near-vertical climb. If done properly, the Guillemot’s speed drops to zero just above the landing ledge, but if not, it has to dive away from the cliff and try again.
Soaring on land: A bird is said to soar when it stays aloft using energy from air currents. Soaring birds do glide and soar while flapping. Soaring is obligatory for many large birds. Slope soaring is the simplest, in which the bird exploits rising air that is deflected upward as the wind blows against a hillside, or some other small obstruction. A gliding bird can gain height by circling in the core, but is carried along by the wind while doing so. At the top of the thermal, the bird glides off in a straight line, losing height until it finds another thermal and repeats the climb. When thermals are marked by cumulus clouds, soaring birds climb as high as 2,000 m or higher. Thermal soaring is the characteristic method of cross-country flight in large soaring birds such as Storks, Pelicans, and migratory Eagles, while many raptors use thermals to soar in search of food. Lee waves are stationary wave systems that form downwind of hills and can be used to soar, but the technique is difficult. Some Geese and Swans are known to use lee waves when migrating. Soaring migration is more advantageous to large birds, in which basal metabolism is only a small fraction of the power required for flapping flight.
Soaring on sea: The vast trade wind zones of tropical oceans have predominantly fair weather, with regularly spaced cumulus clouds. These are the trade wind thermals caused by air mass being convected toward the equator over progressively warmer water. The relatively weaker, trade-wind thermals continue at all hours of the day and night, and provide Frigate Birds with the means to disperse across the oceans without ever alighting on the surface. The middle latitudes, where stronger winds prevail, are the home of the Petrels and Albatrosses. These birds skim with no apparent effort in and out of the wave troughs, sometimes very close to the surface, pulling up to 15 m or so, seldom flapping their wings. A Petrel replenishes its air speed with a kinetic energy push each time it pulls out of the sheltered zone in the lee of a wave, into the unobstructed wind above. As the energy comes from the relative motion between the air and the waves, birds that use this technique are confined to an area just above the sea surface. Albatrosses can also slope-soar in zero wind by gliding along the leading slopes of moving waves. Pelicans and Boobies soar over slopes and cliffs when they come ashore to breed, but use mainly flapping flight at sea, as do Gulls and Auks.
Altitudes: Birds fly near the earth’s surface most of the time, except for soaring species. With the
exception of Frigate Birds, most seabirds spend their entire lives within 30 m of the surface, except when
they come to land, and soar. Radar studies show that passerines fly at heights up to 3,000 m and waders up
to 6000 m during long migration flights. The reduced air density at high altitudes requires birds to fly
faster resulting in some increase in range due to reduced wastage on basal metabolism, caused by the
shorter flight time. The “cross-current” lungs of birds give them the ability of extracting oxygen from
low-density air. Lower air temperatures aloft reduce the need for evaporative cooling.